Rotary drum washers or filters are used in the manufacture of paper products from pulp. A slurry of pulp fibres suspended in water and various chemical additives is formed from wood chips. A sheet of pulp is formed from the slurry on the surface of a large drum by rotating the drum through the pulp slurry. The pulp sheet is then washed and de-watered on the outer surface of the drum, which has a perforated deck. The pulp sheet is then separated from the drum for further processing. Two general types of rotary drum washers are in common use--vacuum washer drums and pressure washer drums. In the vacuum washer drum, low pressure is maintained between an inner and an outer deck to draw the filtrate from the pulp sheet towards the centre of the drum. In the pressure washer drum, the drum is housed in a pressure chamber so that the exterior of the drum is maintained at a higher pressure than the interior, to force the filtrate through the filter deck of the drum. A prior art pressure washer is disclosed in U.S. Pat. No. 3,487,941 issued Jan. 6, 1970 to Rauma-Repola Oy.
In pressure washer drums, the cylindrical filter deck has typically been supported on the axis of rotation by a number of annular spokes at either end of the drum. Such spokes are subjected to considerable forces, due to the combined weight of the drum and the external pressure on the surface of the drum, as well as rotational forces. For this reason the spokes are subject to premature failure, which results in a costly shutdown of the pulp processing facility in order to replace the drum, and expensive repairs. There is therefore a need for an improved design for the pressure washer drum which avoids such premature failure.